Fixation or immobilization of wastes in cement-based materials, commonly referred to as grouts, is an important waste management method. Formulations prepared by mixing the waste material with a dry-solid blend consisting of cement, fly ash and clays are commonly referred to as grouts. Grout properties of importance in waste immobilization variously include the rheologic properties of freshly mixed grouts, the structural stength of cured grouts, the leach properties of cured grouts and the amount of grout phase separation exhibited at various times during curing.
A problem which is encountered in waste disposal is the presence of fluorides in the waste materials. Fluoride acts as a set retarder for cement based grouts and therefore presents a serious problem for immobilization of waste containing fluoride.
Hydrofracture grouts are composed primarily of cementitious phases. Therefore, it is important to understand the reactions that occur during the hydration cement. These reactions are discussed in detail in Stinton, D. P., et al, Characterization of Hydrofracture Grouts for Radionuclide Migration, Oakridge National Laboratory, July 1983. This article shows that a significant amount of calcium hydroxide is generated as a result of the hydration of cementitious phases present in grout materials. In addition, this article also teaches that this calcium hydroxide, sometimes called Portlandite, apparently reacts with carbon dioxide from the air to form carbonates. This article does not address the problem created by fluoride in the waste material to be incorporated in the grout. Moreover, the amount of calcium hydroxide generated by hydration of the cementitious phases does not suffice to inactivate undesirable fluoride present in the waste material and, as a result, fluorides retard or prevent setting of the grout.
Struxness, E. G., et al, Engineering Development of Hydraulic Fracturing as a Method for Permanent Disposal of Radioactive Wastes, Oakridge National Laboratory, Aug. 9, 1968, discusses the initial development of formulations used as hydrofracture grouts. The primary problem addressed by this article is the leaching of radioactive strontium from the grout material. This article teaches that a significant amount of calcium is leached out of the grout material in addition to the salts normally present in the waste material. Presumably, this calcium came for soluble compounds formed in the cement as it set. The analytical data confirmed that the amount of radioactive strontium leached out of the grout material was directly proportional to the amount of calcium which leached from the grout. Accordingly, the article teaches that reduction of the amount of leachable calcium in the grout material is desirable since it will reduce the amount of radioactive strontium leached from the grout.
Additional research has been performed to develop the required cementitious grouts for disposal of low-level radioactive waste. The results of this research are embodied in the following articles: Moore, J. G., et al, Strontium Leachability of Hydrofracture Grouts for Sludge-Slurries, Oakridge National Laboratory, March 1982; Moore, J. G., Development of Cementitious Grouts for the Incorporation of Radioactive Wastes. Part I: Leach Studies, Oakridge National Laboratory, April 1975; and Moore, J. G., Development of Cementitious Grouts for the Incorporation of Radioactive Wastes. Part II: Continuation of Cesium and Strontium Leach Studies, Oakridge National Laboratory, September 1976. These articles deal with the adaptation of cementitious grouts for permanent immobilization of low-level radioactive wastes containing strontium and cesium. These reports recognize that reductions in the amount of cement in the grouts leads to a corresponding reduction in the amount of strontium which leaches out of the grout materials. This phenomenon is directly attributable to the reduction in the amount of leachable calcium present in the grout material as a result of the reduction in the amount of cement present in the material. These articles also found that the amount of strontium leaching out of the grout material could be reduced by addition of stable strontium in combination with calcium ions in the form of calcium chlorides.
Tallent, O. K. et al, Fixation of Waste Materials in Grouts. Part I: Empirical Correlations of Formulation Data, Oakridge National Laboratory, March 1986, explores the properties of cementitious materials used for disposal of waste materials. This publication addresses the variations in critical properties which occur as a result of variations in the composition of the dry-solid materials used to formulate the grout.